Method and apparatus for manufacturing metallic parts by die casting

FIELD OF THE INVENTION

0001The invention relates to a method and apparatus for manufacturing metallic parts, more particularly to a method and apparatus for manufacturing metallic parts by a process involving injection of liquid metal into a mold, including die casting methods.

BACKGROUND OF THE INVENTION

0002Conventional die casting apparatus are classified into cold chamber and hot chamber. In cold chamber die casting apparatus, molten metal is poured into a sleeve which is secured on a die plate and connected to an inlet opening to the mold cavity. Molten metal is injected by a plunger into the die. The molten metal in the sleeve is easily cooled down when it spreads at the bottom of the sleeve as the plunger moves forward slowly to discharge air or gas. Cooled molten metal in the sleeve forms a chilled fraction and semi-solid or solid particles. The chilled fraction and particles are injected into the molding die causing the physical properties of molded parts to be deteriorated.

0003Cooled molten metal increases the viscosity of the molten metal and makes it difficult to fill the mold cavity. Further, it causes blemishes on surface of a molded part. This is a serious problem particularly for magnesium alloys for which the latent heat of solidification is small (smaller than aluminum, lead and zinc). Because of the small latent heat of solidification, magnesium solidifies quickly when it comes in contact with materials having a lower temperature.

0004Hot sleeves have been used, but the heated sleeve is not as hot as liquidus temperature of the metal because the sleeve is connected to a molding die whose temperature has to be below the solidus temperature of the metal. The molding die temperature must be sufficiently below the solidus temperature of the molten metal to produce an adequate solidification rate. That is, a solidification rate which reflects the required time for an operation cycle. Molten metal poured into the sleeve has a substantially higher temperature than the liquidus temperature of the metal to counter the cooling in the sleeve. This is a disadvantage in energy cost for heating.

0005The cold chamber apparatus forms a thick round plate as a part of the casting, often called a biscuit, in the sleeve between a plunger head and an inlet of a die. After the casting is pulled away from the molding dies when the dies are opened, the biscuit is cut away from the casting and recycled. However, sometimes the biscuit is larger than the product. This is a disadvantageous use of metal which has a substantial recycling cost.

0006In hot chamber die casting apparatus, an injection mechanism is submerged in molten metal in a furnace. The temperature of the molten metal to be injected is maintained above its liquidus. The injection mechanism has a shot cylinder with a plunger, gooseneck chamber and a nozzle at the end of thereof. The molten metal is injected through a gooseneck-type passage and through a nozzle into the die cavity without forming a biscuit. This is an advantage of hot chamber die casting apparatus.

0007Another advantage of a hot chamber apparatus over a cold chamber apparatus is the time for an operation cycle. As mentioned above, in cold chamber apparatus, the casting is formed by injecting molten metal into a mold cavity between closed dies and cooling to until the casting is solid. The dies are separated and the molded part is pulled away, lubricant is sprayed onto the opened dies, and the dies are closed again. Then, the dies are ready to start the next operation cycle. The molten metal is poured into the injection sleeve when the molding dies are closed, i.e., when the dies are ready to start the next operation cycle, so that the molten metal does not spill out from the inlet opening of the die because the injection sleeve directly communicates with a die.

0008On the other hand, hot chamber die casting apparatus fill molten metal in the gooseneck and a shot cylinder system by returning an injection plunger to its fill up position. Molten metal is supplied through an opening or fill port on a shot cylinder. While cooling the injected molten metal in the dies, the nozzle is positioned by inclining the gooseneck chamber. The molten metal in the nozzle gooseneck system tends to flow back into the furnace through the fill port on the shot sleeve, reaching a hydrostatic level when the dies are opened. By simultaneously filling molten metal into the gooseneck and a shot cylinder system and cooling injected metal in the closed dies, time for an operation cycle of the hot chamber apparatus is shortened compared with the cold chamber die casting apparatus.

0009However, solidification of the molten metal in the nozzle section of the gooseneck and dripping of molten metal from the nozzle and the cast sprue are problems for hot chamber die casting apparatus. It is known that in hot chamber die casting apparatus a vacuum is created in the injection mechanism when the plunger is withdrawn. However, the vacuum is instantaneously destroyed once the plunger passes the opening or fill port on the shot cylinder supplying molten metal from the furnace because the furnace is at atmospheric pressure. Thus, the molten metal is sucked into the shot cylinder, and the gooseneck and the nozzle are completely filled at the time that the casting is solidified and the dies are separated.

0010There is molten metal in the nozzle for most of the time that the casting is cooling. When the cooling at the tip of the nozzle is properly controlled, it is understood in the industry that the metal in the nozzle tip becomes semi-solid. The formed semi-solid metal works as a plug which prevents molten metal from dripping out of the nozzle when the dies are separated. If the cooling is insufficient, the metal in the tip of the nozzle and the cast sprue is still liquid when the dies are separated and dripping occurs. On the other hand, when too much cooling is applied, the metal in the nozzle tip solidifies and freezes together with the cast sprue. The casting will stick in the stationary die after the dies open.

0011U.S. Pat. Nos. 3,123,875, 3,172,174, 3,270,378, 3,474,875 and 3,491,827 propose creating a vacuum in the gooseneck by return or reverse stroke of the plunger to draw back molten metal from the nozzle and extreme tip of the sprue. These patents disclose mechanisms attached to the shot cylinder and a plunger system so that the created vacuum is kept intact until after the dies have been separated and the solidified casting has been withdrawn from the sprue opening of the stationary die.

0012Problems in the hot chamber die casting apparatus are caused because a heavy injection mechanism is submerged in the molten metal in the furnace. The injection mechanism with a gooseneck chamber and a shot cylinder system is difficult to clean up. It is also difficult to replace worn plunger rings and sleeves. A worn plunger ring and sleeve decreases injection pressure due to leakage and makes shot volume inconsistent in filling the mold cavity. The inconsistent shot volume produces inconsistent molded parts.

0013Die casting apparatus are also classified according to the arrangement of the injection system, that is, horizontal and vertical. In a horizontal die casting apparatus, an injection system is horizontally arranged for horizontally injecting molten metal into molding dies. A vertical die casting apparatus has a vertically arranged injection system for vertical injection of molten metal.

0014Conventional vertical die casting apparatus typically are vertically arranged cold chamber apparatus that have the same advantages and disadvantages of the cold chamber apparatus described above. However, a feature of the vertical die casting apparatus is that the inlet opening for molten metal can be on top of the vertical injection chamber. This arrangement is not applicable to the horizontally arranged apparatus. In U.S. Pat. Nos. 4,088,178 and 4,287,935, Ube discloses machines in which a vertical casting sleeve is pivotally mounted to a base and slants from perpendicular position to accept molten metal. In place of supplying molten metal to the casting sleeve, Nissan Motors discloses in U.S. Pat. No. 4,347,889 a vertical die casting machine in which a vertical casting sleeve moves downward and a solid metal block is inserted. The inserted metal block is melted in the sleeve by an high frequency induction coil. The problem with these apparatus is the complexity of their structure.

SUMMARY OF THE INVENTION

0015One embodiment of the invention relates to an injection molding apparatus comprising a melt furnace, a metal supply system located in the melt furnace, the metal supply system comprising a pump, a first metal inlet from the melt furnace to the metal supply system, a vertical injection mechanism adapted to inject liquid metal into a mold, and a second metal inlet from the metal supply system to the vertical injection mechanism.

0016Another embodiment of the invention relates to an injection molding method comprising providing solid metal into a melt furnace, melting the solid metal into a liquid state in the melt furnace, providing the liquid metal from the melt furnace through a first metal inlet into a metal supply system located in the melt furnace, pumping the liquid metal from the metal supply system through a second metal inlet into a vertical injection mechanism, and injecting the liquid metal from the vertical injection mechanism into a mold located above the vertical injection mechanism.

0017Another embodiment of the invention relates to an injection molding apparatus comprising a melt furnace, a metal supply system comprising a pump and a conduit, a first metal inlet from the melt furnace to the metal supply system, an injection mechanism adapted to inject liquid metal into a mold, a second metal inlet from the conduit of the metal supply system to the injection mechanism, a three way valve located across the conduit, and a valve actuator operatively connected to the valve. The valve actuator is adapted to vertically move the valve to a first vertical position relative to the conduit allow liquid metal to flow from the melt furnace into the conduit, to a second vertical position relative to the conduit to allow liquid metal to flow from the conduit toward the second metal inlet, and to a third position to allow liquid metal to flow from the injection mechanism to a drain.

0018Another embodiment of the invention relates to an injection molding apparatus comprising a melt furnace, a metal supply system comprising a gear pump and a conduit located in the melt furnace, a first metal inlet from the melt furnace to the gear pump, an injection mechanism adapted to inject liquid metal into a die system, and a second metal inlet from the conduit of the metal supply system to the injection mechanism.

0019Another embodiment of the invention relates to a method of injecting liquid metal into a mold comprising providing liquid metal into a vertical injection chamber containing an injection plunger and an injection nozzle, advancing the injection plunger in the injection chamber to drive off air in the injection chamber at a first speed, injecting liquid metal into a mold cavity by advancing the injection plunger in the injection barrel at a second speed greater than the first speed, and retracting the injection plunger to suck back molten or semi-solid metal from at least one of, a sprue of the mold or the injection nozzle tip into the injection chamber.

0020Another embodiment of the invention relates to an injection molding system, comprising, an injection chamber containing an injection nozzle, and a mold system containing a first die, a second die and a sprue bushing in the first die. The injection nozzle and the sprue bushing are shaped such that when the nozzle contacts the sprue bushing, the contact area between the nozzle and the sprue bushing is substantially one dimensional.

0021Another embodiment of the invention relates to a vertical mold system for use with an injection molding apparatus comprising an injection barrel terminating in an injection nozzle, the mold system comprising a lower stationary die, an upper movable die, a mold cavity located in at least one of the lower and the upper die, and a sprue bushing located in the lower die. The mold system further comprises at least one of the following features: (a) an opening in the lower die connected to the sprue bushing, the opening having a diameter that is wider than a diameter of the injection barrel, (b) a shutter plate adapted to cover the injection nozzle when the upper die and the lower die are separated, wherein the shutter plate is located between the upper die and the lower die when the shutter plate covers the injection nozzle, and (c) a shuttle tray adapted to remove a molded part from the mold cavity when the upper die and the lower die are separated, wherein the shuttle tray is located between the upper die and the lower die when the upper and the lower die are separated.

0022Another embodiment of the invention relates to an injection molding method comprising providing material to be injected into a vertical injection barrel terminating in an injection nozzle, closing a vertical mold system comprising a lower stationary die, an upper movable die, a mold cavity located in at least one of the lower and the upper die, a sprue bushing located in the lower die, and an opening located in the lower die connected to the sprue bushing, raising the vertical injection barrel such that the injection nozzle contacts the sprue bushing and at least a portion of the injection barrel is located in the opening in the lower die, injecting the material from the injection barrel into the mold cavity, raising the upper die to open the vertical mold system, moving a shutter plate between the raised upper die and the lower die to cover the injection nozzle, removing a molded part from the mold cavity, spraying the mold cavity with a lubricant after the steps of moving the shutter plate and removing the molded part, moving the shutter plate away from the injection nozzle and out from between the upper and the lower die, and lowering the vertical injection barrel such that the injection nozzle does not contact the sprue bushing.

BRIEF DESCRIPTION OF THE DRAWINGS

0023<figref idref="DRAWINGS">FIG. 1A</figref> is a schematic side view of an injection molding apparatus according to one embodiment of the invention.

0024<figref idref="DRAWINGS">FIG. 1B</figref> is a schematic front view of an injection molding apparatus according to one embodiment of the invention.

0025<figref idref="DRAWINGS">FIG. 2A</figref> is a schematic side view of an injection molding apparatus according to one embodiment of the invention illustrating a method of injection molding according to one embodiment of the invention.

0026<figref idref="DRAWINGS">FIG. 2B</figref> is a front view of an injection molding apparatus according to one embodiment of the invention illustrating a method of injection molding according to one embodiment of the invention.

0027<figref idref="DRAWINGS">FIGS. 3A-3C</figref> are schematic views of a three-way valve according to one embodiment of the invention illustrating A) a first setting, B) a second setting and C) a third setting of the valve.

0028<figref idref="DRAWINGS">FIG. 4A</figref> is a schematic view of a vertical injection barrel and nozzle according to one embodiment of the invention.

0029<figref idref="DRAWINGS">FIG. 4B</figref> is a close up view of a nozzle according to a comparative example.

0030<figref idref="DRAWINGS">FIGS. 5A</figref>, <b>5</b>B and <b>5</b>C are schematic views of a shutter mechanism according to one embodiment of the invention including A) side, B) top and C) rear views.

0031<figref idref="DRAWINGS">FIGS. 6A</figref>, <b>6</b>B and <b>6</b>C are schematic views illustrating the method of using the shutter mechanism of <figref idref="DRAWINGS">FIG. 5A</figref> including A) front, B) side and C) detailed side views.

0032<figref idref="DRAWINGS">FIGS. 7A</figref>, <b>7</b>B and <b>7</b>C are schematic views of a mold system according to an embodiment of the invention including A) side, B) front, C) side detail, and D) side detail with open mold views.

0033<figref idref="DRAWINGS">FIGS. 8A</figref>, <b>8</b>B and <b>8</b>C are schematic views of an embodiment of the invention having a gear pump including A) side, B) front and C) detail views.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

0034As illustrated in <figref idref="DRAWINGS">FIGS. 1A</figref>, <b>1</b>B, <b>2</b>A and <b>2</b>B, one embodiment of the present invention is a vertical die casting apparatus with a horizontal die arrangement. The die casting apparatus is comprised of a furnace <b>1</b>, a casting metal supply system <b>2</b>, a vertical injection mechanism <b>3</b> and a horizontally arranged mold or die system <b>4</b>.

0035The furnace has a heating chamber <b>11</b> and an opening <b>12</b> that provides access for a gas flame or other heat-supplying means. To maintain the casting metal <b>16</b> in a liquid state, a melting pot <b>13</b> is mounted in the heating chamber <b>11</b>. The melting pot <b>13</b> is preferably separated into two receptacles, A and B, by means of partition <b>14</b>. The melting pot <b>13</b> is covered by an insulated metal plate <b>55</b>. In addition, it is preferable for metals which are easily oxidized, such as magnesium alloys, to introduce inert gas such as argon or SF₆. The receptacle A is for melting metal ingots or pellets, supplied through an opening <b>17</b> covered by door <b>19</b>. Through an opening <b>15</b> in the lower part of the partition <b>14</b>, clean molten (i.e. liquid) metal <b>16</b> passes to the receptacle B, where the molten metal <b>16</b> is maintained at a temperature preferable for casting of the metal, such as above the liquidus temperature. Alternatively, the partition may comprise a mesh filter which allows liquid but not solid metal to pass through it.

0036The temperature of the molten metal <b>16</b> is measured by a thermocouple. Heat output of the heat-supply means is adjusted according to feedback of the measured temperature. The level of the molten metal <b>16</b> in the melting pot <b>13</b> is determined by a level sensor <b>18</b> and maintained in a certain range by controlling the volume of metal supplied through the opening <b>17</b>. Preferably, the level of molten metal <b>16</b> is controlled by pulling down a suspended ingot into the melt, by moving a conveyer supplying ingots or pellets over the opening <b>17</b> for a predetermined time or by hand feeding solid metal into opening <b>17</b>, in response to a signal from the level sensor <b>18</b>.

0037The casting metal supply system <b>2</b> is attached to a plate <b>20</b> and comprises a metering sleeve <b>21</b>, in which a metering plunger <b>23</b> is inserted, a three-way valve <b>22</b>, a conduit <b>38</b> and a conduit <b>24</b>, which corresponds to a gooseneck. The lower part of the system <b>2</b> is submerged in the molten metal <b>16</b> so as to keep the molten (i.e., liquid) metal <b>16</b> in the metal supply system <b>2</b> at the same temperature as molten casting metal <b>16</b> in the melting pot <b>13</b>. Therefore, the level of the casting metal <b>16</b> in the receptacle B in the melting pot <b>13</b> should be well above the full up position of the metering plunger <b>23</b> in the plunger sleeve <b>21</b>.

0038Functions of the three way valve <b>22</b> are schematically shown in FIG. <b>3</b>. Preferably, the three way valve <b>22</b> comprises a tube containing three passages <b>39</b>A, <b>39</b>B and <b>39</b>C that is adapted to move perpendicular to a metal flow direction in the adjacent conduit(s) <b>24</b>, <b>38</b>. However, the valve <b>22</b> may have any other suitable valve structure and configuration. The first passage <b>39</b>A is preferably parallel to the metal flow direction in the first <b>38</b> and the second <b>24</b> conduits to connect parallel portions of the first and the second conduits to each other. The second passage <b>39</b>B preferably comprises at least one portion that is inclined by 1 to 90 degrees with respect to the metal flow direction in the first conduit <b>38</b>. For example, passage <b>39</b>B may be a diagonal passage inclined by 20 to 70 degrees. Passage <b>39</b>B connects the first metal inlet <b>40</b> to the first conduit <b>38</b> which is operatively connected to the pump <b>23</b>. The third passage <b>39</b>C comprises at least one portion that is inclined by 1 to 90 degrees with respect to the metal flow direction in the second conduit <b>24</b>. For example, the third passage <b>39</b>C may be a passage having a horizontal and a vertical portion. Passage <b>39</b>C connects a drain to the second conduit <b>24</b>.

0039The three-way valve changes passages for the casting metal. Initially, (<figref idref="DRAWINGS">FIG. 3B</figref>) the metering plunger <b>23</b> is at the full up position with opening <b>27</b> located above the plunger and opening <b>28</b> below the plunger. When the metering plunger <b>23</b> descends as shown in <figref idref="DRAWINGS">FIG. 3A</figref>, molten metal <b>16</b> flows in over the metering plunger through both openings <b>27</b>, <b>28</b>. When the metering plunger <b>23</b> moves upward, molten metal <b>16</b> on top of the metering plunger <b>23</b> is lifted and then flows out from both openings, finally leveling with molten metal <b>16</b> in the melting pot <b>13</b>.

0040Due to the flow from the both openings <b>27</b>, <b>28</b>, the metering plunger <b>23</b> is heated up to the same temperature as the molten metal <b>16</b> in the melting pot <b>13</b>. Thus, the temperature of the metering plunger <b>23</b> does not affect the temperature of molten metal <b>16</b> in the metering sleeve <b>21</b>. Further, heaters are attached around the conduit <b>24</b> above the level of the molten metal <b>16</b> to keep the metal therein molten at a temperature chosen considering casting performance. Preferable heaters for the conduit <b>24</b> are coil heaters or sheathed heaters.

0041In the first setting of the three-way valve <b>22</b>, a valve actuator <b>26</b> lowers the three-way valve <b>22</b> to a first position so that a first passage <b>39</b>A fluidly connects the plunger sleeve <b>21</b> to the injection barrel <b>31</b> via a first conduit <b>38</b>, a second conduit <b>24</b> and a connecting port <b>37</b> to allow the molten metal to flow from the metering plunger toward an opening <b>33</b> in the injection barrel <b>32</b>. The metering plunger <b>23</b> is then lowered to force metal from sleeve <b>21</b> through conduit <b>38</b>, valve <b>22</b>, conduit <b>24</b> and opening <b>33</b> into chamber <b>31</b>. After the metal is provided to chamber <b>31</b>, the valve actuator <b>26</b> is lifted to a second position until the second passage <b>39</b>B connects an inlet port <b>40</b> to the first conduit <b>38</b> to allow molten metal to flow from the melting pot <b>13</b> through opening <b>40</b> into the sleeve <b>21</b>. When the metering plunger <b>23</b> is withdrawn, suction is created, drawing molten metal <b>16</b> from the melting pot <b>13</b> to the metering sleeve <b>21</b>.

0042During normal operation, only the first two passages <b>39</b>A, <b>39</b>B are used. However, if it becomes necessary to remove the casting metal supply system <b>2</b> to perform maintenance, the three-way valve <b>22</b> may be operated in the third position. In this position, the second conduit <b>24</b> is connected to a drain <b>57</b>. In this manner, molten metal <b>16</b> in the injection barrel <b>31</b> and the second conduit <b>24</b> can be emptied into the melting pot <b>13</b>.

0043The injection mechanism <b>3</b> is attached to a base plate <b>30</b> on which the plate <b>20</b> is also fixed supporting the casting metal supply system <b>2</b>. As the injection mechanism <b>3</b> and the casting metal supply system <b>2</b> are rigidly attached to the same base plate <b>30</b>, these two components move up and down simultaneously without moving the melt furnace <b>1</b>. While two plates <b>20</b>, <b>30</b> are illustrated as rigidly attaching components <b>2</b> and <b>3</b> together, other attaching devices may be used instead. For example, one or more plates, rods or clamps may be used to attach components <b>2</b> and <b>3</b> to each other. Therefore no bending force is applied to the conduit <b>24</b> and material for the metal supply system <b>2</b> can be selected from various materials including ceramics suitable for light metal injection, such as magnesium or aluminum injection. The injection mechanism <b>3</b> is comprised of an injection barrel <b>31</b> with a connection port <b>37</b>, an injection plunger <b>32</b> located in the injection barrel <b>31</b> and an injection nozzle <b>35</b> on the top of the injection barrel <b>31</b>. The casting metal <b>16</b> is poured into the injection barrel <b>31</b> through a metal inlet opening <b>33</b> connected to the conduit <b>24</b> at the connection port <b>37</b>. The connection port <b>37</b> declines to the conduit <b>24</b> so that in an emergency, casting metal <b>16</b> in the barrel <b>31</b> is drained back to the melting pot <b>13</b> through the three-way valve <b>22</b>. This is illustrated in FIG. <b>3</b>C.

0044As shown in <figref idref="DRAWINGS">FIGS. 4A and 4B</figref> the injection barrel <b>31</b> is heated by heaters <b>311</b><i>a, b, c </i>and <i>d </i>to maintain the injection barrel <b>31</b> above liquidus temperature of the metal to be injected. In addition, a heater <b>311</b><i>e </i>heats the injection barrel connection port <b>37</b>. The heaters <b>311</b><i>a, b, c, d </i>are divided into sections so that each heater may be maintained at a different temperature and the poured casting metal <b>16</b> may be maintained at the most preferable temperature for injection. Each heater is independently controlled in response to a signal from a corresponding thermocouple <b>312</b><i>a, b, c </i>and <i>d </i>inserted in wall of the injection barrel <b>31</b> and the nozzle <b>35</b>. The injection barrel connection port heater <b>311</b><i>e </i>is controlled by thermocouple <b>312</b><i>e. </i>

0045The injection mechanism <b>3</b> and the injection plunger <b>32</b> are preferably actuated by a hydraulic cylinder <b>74</b> and a hydraulic piston cylinder <b>75</b> respectively. However, any means capable of raising the injection mechanism <b>3</b> and the injection plunger <b>32</b> may be used. Exemplary devices include, but are not limited to, mechanical, electrical, and pneumatic devices and combinations thereof.

0046It is preferable to maintain the nozzle temperature above liquidus of the metal. The nozzle <b>35</b>, heated above the liquidus, is cooled due to heat conduction, especially when the nozzle <b>35</b> is docked with the sprue bushing <b>41</b>, which has the same temperature as the dies <b>42</b>, <b>43</b> of the die system <b>4</b>. The die temperature is much lower than the solidus temperature of the metal. This is because the casting metal has to solidify in the mold or die cavity <b>44</b> quickly for high productivity. Therefore, the nozzle <b>35</b> is cooled due to heat conduction from the nozzle <b>35</b> to the dies <b>42</b>, <b>43</b> via the sprue bushing <b>41</b>. The cooling rate of the nozzle <b>35</b> corresponds to rate of heat loss transferred from the nozzle <b>35</b> to the dies <b>42</b>, <b>43</b>. This is determined by heat gradient, area in contact and duration of heat transfer. The temperature of the nozzle <b>35</b> is determined as one of casting conditions of the metal while that of the dies <b>42</b>, <b>43</b> are determined mainly by productivity. The primary difference is the gradient of temperature. Therefore, the contacting area between the nozzle <b>35</b> and the sprue bushing <b>41</b> should be minimized by preferably contacting in line <b>85</b>A as shown in <figref idref="DRAWINGS">FIG. 4A</figref> instead of contacting in a face <b>85</b>B as shown in FIG. <b>4</b>B. In other words, the injection nozzle <b>35</b> and the sprue bushing <b>41</b> should be shaped such that when the nozzle contacts the sprue bushing, the contact area between the nozzle and the sprue bushing is substantially one dimensional (i.e., a line or a ring having a width of 1 mm or less in a direction of the length of the nozzle). The difference in radius and angle of the nozzle head <b>35</b> and the sprue bushing <b>41</b> should be not less than 1 mm and 1 degree respectively, and docking time of the two parts should be as short as possible.

0047A die or mold system <b>4</b> is located over the injection mechanism <b>3</b>. In <figref idref="DRAWINGS">FIGS. 1A and 4A</figref>, the die system <b>4</b> is horizontally located in which a fixed die <b>42</b> and a movable die <b>43</b> are secured on each die block. A sprue bushing <b>41</b> is fixed on each die as <b>41</b><i>a </i>and <b>41</b><i>b. </i>A die or mold cavity <b>44</b> is preferably engraved on the fixed die <b>42</b> and an ejector plate with knockout pins (not shown) is attached to rear side of the movable die <b>43</b>. The ejector plate is moved forward and retracted by a hydraulic cylinder (not shown).

0048Under the sprue bushing <b>41</b>, a shutter <b>6</b> is attached and secured on the fixed die <b>42</b>. Details of the shutter <b>6</b> are depicted in <figref idref="DRAWINGS">FIGS. 5A-5C</figref>. The shutter <b>6</b> includes a shutter plate <b>61</b>, which has a fitting <b>62</b> into which a guide bar <b>63</b> is inserted. The shutter plate <b>61</b> is actuated by a cylinder <b>64</b> connected to the fitting <b>62</b>. The shutter plate <b>61</b> stays back during a stage in which the injection barrel <b>31</b> is up and the sprue bushing <b>41</b> and the injection nozzle <b>35</b> are in contact. When the injection barrel <b>31</b> is pulled downward and the nozzle <b>35</b> is detached from the sprue bushing <b>41</b>, the shutter <b>6</b> is actuated to slide forward and stops at a position over the nozzle <b>35</b>. The shutter <b>6</b> protects the nozzle <b>35</b> from damage by falling solidified metal particles or mist of lubricant sprayed to the dies while the dies are separated and in an open position.

0049The furnace <b>1</b> and the injection mechanism <b>3</b> with the casting metal supply system <b>2</b> fixed on the base plate <b>30</b> are placed on a sliding plate <b>5</b> shown in FIG. <b>1</b>A. As the die height, or thickness of a pair of dies, varies depending on the size of a casting article, the position of the nozzle <b>35</b> on the top of the injection barrel <b>31</b> is adjusted by sliding the plate <b>5</b> in alignment with the receiving sprue bushing <b>41</b> on the dies <b>42</b>, <b>43</b>.

0050The operation of the injection molding apparatus of the preferred embodiment is explained stepwise as follows. In the following description, the operation begins when injection of the casting metal is completed.

0051In the first phase of the casting operation, the dies <b>42</b> and <b>43</b> are closed and the nozzle <b>35</b> is docked with the sprue bushing <b>41</b> on the dies <b>42</b>, <b>43</b>. The injection plunger <b>32</b> is in an upper most position and blocks the opening <b>33</b> such that no metal flows between the injection barrel <b>31</b> and the metal supply system <b>2</b>. As soon as the molten metal <b>16</b> in the dies (particularly the metal in the gate where the cavity <b>44</b> is the thinnest) has had time to solidify (typically a second or less for magnesium alloys), the injection plunger <b>32</b> quickly retracts to an intermediate position in the injection barrel <b>31</b>, sucking molten or semi-solid metal in the sprue <b>41</b> and the nozzle opening <b>36</b> back into the injection barrel <b>31</b>. By sucking metal in the nozzle tip back, clogging of the nozzle <b>35</b> or formation of a plug is prevented. Further, any semi-solid metal which is sucked back will be remelted in the injection barrel <b>31</b>. This is significant for the present apparatus as it allows air in the injection barrel <b>31</b> to vent from the opening <b>36</b>.

0052In order to avoid further cooling of the nozzle <b>35</b>, immediately after sucking, the injection barrel <b>31</b> is actuated downward. The injection plunger <b>32</b> continues retracting at a reduced speed compared to the suck back speed until a head of the injection plunger <b>32</b> comes just above the opening <b>33</b> to conduit <b>24</b> on the lower part of the injection barrel <b>31</b>, such that the opening <b>33</b> remains blocked or closed by the injection plunger <b>32</b>. Alternatively, the injection plunger <b>32</b> may remain at the intermediate position in the barrel <b>31</b> after performing sucking back the metal, until the plunger <b>32</b> is moved down below opening <b>33</b> to expose the opening <b>33</b> to receive molten metal from the metal supply system <b>2</b>.

0053The distance of retraction of the injection barrel <b>31</b> is preferable less than 10 mm, for which distance the metal supply system <b>2</b> also retracts in the pot <b>13</b>. It is further preferable that the distance of movement should be less than 5 mm, as solidified metal tends to deposit in the zone where the submerged part of the metal supply system <b>2</b> goes up from the level of molten metal <b>16</b>.

0054The shutter plate <b>61</b> is then actuated and moves to a position over the nozzle <b>35</b> to protect the nozzle head from molten metal dripping from the dies. The nozzle temperature begins to rise because the heat conduction has ceased and because the heater <b>311</b><i>a </i>for the nozzle <b>35</b> is on, having sensed the decreased temperature at the thermocouple <b>312</b><i>a </i>inserted into the nozzle head. The nozzle temperature returns to the set temperature before the next injection cycle begins. The position of the sensing tip of the thermocouple is preferably located to detect the actual nozzle temperature. The sensing tip should be as close to the nozzle opening <b>36</b> as possible, as shown in FIG. <b>4</b>B. This procedure is another advantageous aspect of the present invention.

0055In the second phase, the casting in the die cavity is cooled and solidifies. The time for solidification is from 1 or less seconds to about 10 seconds depending on the size and thickness of the article being cast. Then, the dies are separated and molded article on the moving die <b>43</b> is ejected onto a chute or removed by a robot. The die face is cleaned and lubricant is sprayed on the dies <b>42</b>, <b>43</b>.

0056During this period of time, the supply system <b>2</b> is at least partially, and preferably fully submerged in molten casting metal <b>16</b> and the molten casting metal <b>16</b> is sucked into the metering sleeve <b>21</b> by withdrawing the metering plunger <b>23</b> up to the full up position. The casting metal <b>16</b> comes into the plunger sleeve <b>21</b> through the three-way valve <b>22</b> communicating with the melting pot as shown in FIG. <b>3</b>B. The suctioning of the casting metal <b>16</b> is completed when the metering plunger <b>23</b> passes an opening <b>28</b> on upper part of the metering plunger sleeve <b>21</b> and, therefore, pressure in the metering sleeve <b>21</b> becomes atmospheric pressure. Without opening <b>28</b>, the present apparatus works, but with the opening it is assured that no air is left in the metering sleeve <b>21</b>.

0057Then, the three-way valve <b>22</b> closes the passage <b>39</b>B communicating with the melting pot <b>13</b>, and connects the sleeve <b>21</b> to the conduit <b>24</b> via passage <b>39</b>A, as shown in FIG. <b>3</b>A. The injection plunger <b>32</b> moves downward and opening <b>33</b> is opened to receive casting metal <b>16</b> from the supply system <b>2</b>, as shown in FIG. <b>2</b>A. The casting metal <b>16</b> is forced into an injection barrel <b>31</b> by pushing down the metering plunger <b>23</b> to a desired distance corresponding to a volume required for a shot. The precise metering of casting metal <b>16</b> is another advantage of the present apparatus, because it reduces or eliminates burrs around castings caused by an excessive volume of casting metal <b>16</b> and pressure in the die cavity <b>44</b>. Burrs on the casting reduce reproducibility and reliable operation, because burrs unexpectedly stuck to the dies <b>42</b>, <b>43</b> cause troublesome leakage of casting metal <b>16</b>. The burrs may also cause dents or deformation on the parting face of the dies <b>42</b>, <b>43</b>, leading to thicker and larger burrs. Without burrs, machining costs for articles after molding are reduced.

0058Precise metering is achieved in that the metal supply system <b>2</b> of the present apparatus preferably operates without high pressure and without high speed in forcing casting metal <b>16</b> into the injection barrel <b>31</b>. High pressure and high speed are the reasons that a plunger pump in a hot chamber die casting machine is heavy and inaccurate. Immediately after metering of the casting metal <b>16</b> is completed, the injection plunger <b>32</b> slowly moves upward and stops when the inlet opening <b>33</b> is closed off.

0059In the third phase, the molding dies <b>42</b>, <b>43</b> are engaged and set into a closed position. The shutter <b>6</b> moves backward and the injection barrel <b>31</b> is pushed upward by a hydraulic cylinder <b>74</b> until the nozzle <b>35</b> firmly docks onto a sprue bushing <b>41</b> on the dies <b>42</b> and <b>43</b>. The metal supply system <b>2</b> is at least partially lifted from the melting pot <b>13</b> because the system <b>2</b> is attached to the injection barrel <b>31</b> by plate <b>30</b>. Then the injection plunger <b>32</b> is actuated upward slowly by a hydraulic system <b>75</b> to expel the air over the casting metal <b>16</b> from the nozzle opening <b>36</b> and to vent from an air vent (not shown) engraved on the dies <b>42</b>, <b>43</b> through die cavity <b>44</b>. The position of the injection plunger <b>32</b> at the time the air in the injection barrel <b>31</b> is exhausted is predetermined by calculating from the dimensions of the injection barrel <b>31</b> and the metered volume of casting metal <b>16</b>.

0060Alternatively, the air may be expelled from the injection barrel before the nozzle docks with the sprue bushing <b>41</b> in order to reduce the process time for making a molded part. Preferably, the air is expelled from the injection barrel <b>31</b> at the same time as another process step is being carried out. For example, the injection plunger <b>32</b> may be actuated upward slowly to expel the air over the casting metal <b>16</b> from the nozzle opening <b>36</b> in the second phase of the process when the dies <b>42</b>, <b>43</b> are in the open position and the molded part is being removed and the dies are being cleaned and lubricated. The distance of upward movement of the injection barrel, the volume of the injection barrel, the amount of metal metered into the injection barrel and the position of the injection barrel and the injection plunger are programmed and controlled by a control system, such as a computer, in order to reduce or prevent metal from overflowing from the nozzle opening <b>36</b> while air is being expelled.

0061In a prior art method, a plug clogging the nozzle is shot out toward die cavity and the compressed air is injected into the die cavity along with casting metal. Not only the plug, but also air caught in the casting metal reduces the cosmetic and physical properties of the article being cast. Thus, the sucking back process described with respect to the first stage above is advantageous because it avoids introducing the plug and air into the cavity <b>44</b>. At the predetermined position where the air in the injection barrel <b>31</b> is exhausted, the speed of the injection plunger <b>32</b> is accelerated instantly and the casting metal <b>16</b> is injected into the die cavity <b>44</b>. The injection plunger <b>32</b> is then decelerated and stopped. The deceleration of the injection plunger <b>32</b> toward the end of injection prevents the injection plunger <b>32</b> from bumping against upper end of the injection barrel <b>31</b>.

0062Though the volume of casting metal <b>16</b> is precisely metered and the temperature thereof is also strictly controlled, the position of the injection plunger <b>32</b> at the end of injection may fluctuate due to unexpected factors such as (1) friction increase caused by precipitation of impurities in the molten metal on the surfaces of the injection barrel <b>32</b> and/or the plunger or (2) injection pressure loss by leakage through piston rings (not shown). In the present apparatus, the position of the injection plunger <b>32</b> is preferably detected or measured by a potentiometer secured on the injection plunger rod. When the injection is completed, the detected injection plunger position is compared with the desired normal position and the difference is transformed through a calculation circuit into a volume of casting metal. Then, the signal is transmitted to the metal supply system <b>2</b> as a distance for descending the metering plunger <b>23</b> and/or as a distance for descending the injection plunger <b>32</b>. The downward movement of the plunger <b>23</b> precisely meters the amount of the casting metal volume provided into the injection barrel <b>31</b>.

0063Another embodiment of the present invention includes a vertical die casting apparatus with a vertical die arrangement. As illustrated in a drawing of <figref idref="DRAWINGS">FIGS. 6A-C</figref>, a furnace <b>1</b>, a casting metal supply system <b>2</b> and a vertical injection mechanism <b>3</b> are the same as in the previous embodiment. In this embodiment, a die system <b>4</b> is arranged vertically and a sprue bushing <b>41</b> is inserted to a stationary lower die <b>42</b>. An ejector plate with knockout pins is attached to a movable upper die <b>43</b> above the stationary lower die <b>42</b>. The injection barrel <b>31</b> moves up and down through an opening <b>46</b> on a die block <b>45</b> while the diameter of the opening <b>46</b> in the die block is larger than the injection barrel <b>31</b>. A shutter <b>6</b> is located behind the dies <b>42</b> and <b>43</b>, and a shuttle tray <b>7</b> is located on one side of the dies <b>42</b>, <b>43</b>. The locations of the shutter <b>6</b> and tray <b>7</b> may be reversed if desired. The operation of the apparatus of this embodiment is same as that of the apparatus of the previous embodiment with a horizontal die arrangement in <figref idref="DRAWINGS">FIGS. 1A and 2A</figref>. When the dies are opened, the molded article is separated on the movable die <b>43</b> and the shutter <b>6</b> and the tray <b>7</b> are actuated forward. The shutter <b>6</b> protects the nozzle head from the mist of lubricant sprayed. The shuttle tray <b>7</b> receives the molded article ejected by the knockout pins and the article is removed from the die area. In this embodiment, the sprue formed is larger than that in the embodiment with horizontally arranged dies.

0064Another embodiment of the present invention is shown in <figref idref="DRAWINGS">FIGS. 7A-D</figref>, where the injection barrel <b>31</b> reaches a sprue bush <b>41</b> secured on a die face of the stationary die <b>42</b>. In this embodiment, the length of the sprue is shortened compared with the embodiment in FIG. <b>6</b> and thus, the volume of a formed sprue is reduced.

0065An injection molding method using the vertical die system shown in <figref idref="DRAWINGS">FIGS. 6A and 7A</figref> is a follows. Molten metal is provided into the vertical injection barrel <b>31</b> terminating in an injection nozzle <b>35</b>. The vertical mold system is closed and the vertical injection barrel is raised, such that the injection nozzle <b>35</b> contacts the sprue bushing <b>41</b> and at least a portion of the injection barrel <b>31</b> is located in the opening <b>46</b> in the lower die <b>42</b>. The metal is injected from the injection barrel <b>31</b> into the mold cavity. The injection barrel <b>31</b> is lowered such that the injection nozzle <b>35</b> does not contact the sprue bushing <b>41</b>. The upper die <b>43</b> is raised to open the vertical mold system. The shutter plate <b>61</b> is moved between the raised upper die <b>42</b> and the lower die <b>42</b> to cover the injection nozzle <b>35</b>, as shown in FIG. <b>6</b>C. The shuttle tray <b>7</b> is provided between the raised upper die <b>43</b> and the lower die <b>43</b> before or after the shutter plate <b>61</b> is moved between the die, as shown in <figref idref="DRAWINGS">FIGS. 6C and 7D</figref>. The knock out pins are extended in the upper die <b>43</b> to disengage the molded part from the upper die <b>43</b> and to drop the molded part onto the shuttle tray <b>7</b>. The molded part is removed from the mold cavity by removing the shuttle tray <b>7</b> containing the molded part out from between the upper and the lower die (i.e., to the side of the die as shown in FIG. <b>7</b>A. The mold cavity is cleaned and sprayed with a lubricant after the steps of moving the shutter plate and removing the molded part. Then, the shutter plate <b>61</b> is moved away from the injection nozzle and out from between the upper <b>42</b> and the lower die <b>43</b> (i.e., it is moved behind the die), as shown in FIG. <b>6</b>B. The dies <b>42</b>, <b>43</b> are closed and are ready for the next injection step.

0066Still another embodiment of the invention is illustrated in <figref idref="DRAWINGS">FIGS. 8A-8C</figref>. In this embodiment, the casting metal supply system <b>2</b> comprises a gear pump <b>221</b> rather than the plunger pump of the previous embodiments. In addition, this embodiment does not use the three-way valve <b>22</b> of the previous embodiments. In a preferred aspect of this embodiment, the gear pump <b>221</b> is powered by a motor <b>223</b>. Power is transferred to the gear pump <b>221</b> by use of a motor rod <b>222</b>. To supply molten metal <b>16</b> to the injection barrel <b>31</b>, the gear pump <b>221</b> is turned on. When sufficient casting metal is supplied to the injection barrel <b>31</b>, the gear pump <b>221</b> is simply turned off. Because there is no need to fill a metering sleeve <b>21</b> in this embodiment, there is no need for a three-way valve <b>22</b>.

0067It should be noted that elements of the apparatus of the above described embodiments may be used interchangeably in any suitable combination. For example, the gear pump <b>221</b> of <figref idref="DRAWINGS">FIG. 8A</figref> may be used together with a vertical die arrangement of <figref idref="DRAWINGS">FIGS. 6A and 7A</figref>.

0068The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The drawings and description were chosen in order to explain the principles of the invention and its practical application. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.